Modern aircraft design recognizes conflicting priorities between higher speed and lower speed operations. Aircraft for low speed flight differ markedly from those intended for high speed flight, and one type may rarely be useful for the other. Historically, to obtain higher speed requires higher power, and high powered aircraft use a lot of fuel. Fast aircraft generally require long paved runways. Likewise, to shorten takeoff and landing distances, faster aircraft demand complex design, controls, and operation. Fast, but efficient aircraft—those having a minimum total of induced drag, surface drag (also known as friction drag or parasitic drag), and for supersonic aircraft, wave drag—also cost more because they are sensitive to size, weight, and incorporation of all the mechanisms used to configure the aircraft for low speed operation, such as when landing. This mandates more expensive design and materials. Comprehensive solutions targeting such problems at their most fundamental levels are of great economic value, but until the present, to obtain lower drag in higher speed operation remains an expensive process filled with compromise.
Two goals common to aircraft invention are the improvement of handling, especially at low speeds, and the reduction of drag. However, improved handling is frequently obtained at the cost of additional drag. Thus, aircraft types offering good handling at low speeds tend to have lower top speeds. While reductions in drag allow a reduction in power requirements and fuel consumption, increases in available payload or range, or corresponding reductions in weight, designers have to choose between the types of drag they can reduce, or accept both compromise and high costs. At low speeds, encountered during takeoff and landing and while maneuvering in airport traffic patterns, surface drag reductions offer little benefit. Indeed, highly streamlined aircraft frequently handle poorly at low speeds and are further disadvantaged by the time or distance needed to slow the vehicle down. At higher speeds, surface drag caused by minor variations and imperfections becomes critical. On the other hand, lower induced drag greatly improves climb performance and payload capacity for a given available power, improving range and fuel economy well beyond whatever nominal savings are shown in cruising flight. Lower air density at high altitudes rapidly demonstrates the value of designing for lower induced drag, because true airspeeds increase in thinner air. Lower induced drag improves high altitude flight, leading to benefits in high speed operation. This makes the reduction of induced drag significant for most aircraft, yet, aside from soaring applications, low induced drag is uncommon among low speed aircraft and rare among high speed aircraft. Thus a pressing need is improved low speed handling in an aerodynamically clean aircraft also having low induced drag.
According to both classical aerodynamic theory and experience, increasing wingspan lowers induced drag. However, all aircraft seeking greater payload or economy through higher efficiency quickly reach limits for material strengths and airport infrastructure, which constrain wingspan. Therefore, a goal of many aircraft designers is to obtain the induced drag reduction of greater wingspan by means of technology having similar effect. Unfortunately, many such efforts are not practical. Some prior art lowers induced drag by marginal amounts, yet adds to total drag, weight, and complexity to such a degree that their net overall value is debatable. Simultaneous reduction of induced drag and surface drag demands an entirely new approach.
Consequently, aircraft capable of high speed operation remain high powered. They often require flaps, slats, or other high-drag means of lift augmentation even to operate at low speeds.
High costs of safely achieving such efficiency-promoting goals as laminar flow and pressure seal of the aircraft flight surfaces mean that fuselage drag remains the easiest target for compromise, and in a typical high speed aircraft, cabin volume is minimized. This negatively impacts the passenger experience and lowers utility. At the same time, efficiency losses of the smallest magnitude represent millions of dollars in transportation fuel costs annually. Equivalent performance at lower fuel consumption is a need having extreme economic benefits.
Another goal of aircraft invention is greater safety. Crash prevention, short field and unimproved runway operation are objectives unfulfilled by the majority of prior art, especially among faster aircraft. Historically, stalls and stall/spins are the major cause of aircraft accidents and are typically deadly when they occur in close proximity to the ground or structures. A factor contributing to stall related accidents is the erroneous belief that stall is a function of airspeed; that stalls do not happen above certain “speeds”. It does not help that “stall speed” is a term that permeates aviation, even though the correct understanding is widely known. Aircraft that do not stall thus often represent an ideal objective, but a rare reality. Likewise, improvement in air transportation systems require aircraft able to operate safely at both lower and higher speeds than at present, such that safer future aircraft may be defined in part by the smaller size of airports or private airfields needed to handle their operations. Growth in personal air vehicle initiatives is even more dependent upon safe low speed handling characteristics, reduced noise, and improved ease of operation. Fast aircraft that can fly slowly while remaining fundamentally incapable of departure from fully controlled flight thus represent a key to distributed transportation solutions. For commercial aviation, at the other end of the size scale, dangerously powerful vortex created in the wake of very large transport aircraft represents both hazard and inefficiency. Invention that reduces wake vortex for fuel economy also promotes safer interaction of large planes with other aircraft.
The efficiency of an aircraft can be stated in terms of its lift-to-drag ratio, or L/D, as is well known within the art. All aircraft operate over a range of L/D based upon apparent fluid viscosity of the air and their flight configuration. Thus an aircraft flying at its optimum speed will display a higher L/D than at speeds faster or slower. Fundamentally, the L/D achieved at specific weight, speed, and power provides a metric known as “specific resistance.”
By studying the specific resistance of various forms of transportation, Gabrielli and von Karman indirectly established the approximate maximum achievable L/D ratio at a given airspeed. This theoretical limit has become known widely as the Gabrielli-von Karman limit, referenced hereinafter as GvK. (Gabrielli, G., and von Karman, T., “What Price Speed?” Mechanical Engineering, Vol. 72, October, 1950) Due to the complexity of calculation involving fluid viscosity, such information is difficult to arrive at directly. The larger perspective of this work reveals the extent of underlying friction and viscosity losses due to motion involving resistive mediums, such as air or water. Experience and subsequent enlargement of the concept have revealed that while technology improvements can be expected to push the achievable limits toward greater efficiency over time, there is presently a very large opportunity for improvement.
Fabio Goldschmied restated specific resistance as an aircraft's L/D under full power at maximum weight, which can be revealed by means of calculation using performance data. His Aerodynamic Efficiency Index (AEI) published in 1987 is a number that allows all aircraft to be compared meaningfully in terms of their specific efficiency. A graph of the AEI of a representative spectrum of aircraft, in which the theoretical limit is also shown, reveals this same vast opportunity for improvement noted earlier; thus also the failure of invention to address the true causes of efficiency loss in prior art.
Unpowered aircraft, being reliant upon their aerodynamic efficiency alone, are often considered the ultimate expression of drag reduction. Indeed, modern composite sailplanes have demonstrated L/D ratios of more than 60:1. Surprisingly, however, this result hardly approaches the GvK limit due to the slow speeds at which it is obtained. Yet typical powered aircraft, which fly faster, seldom achieve a third of that figure. Worse yet, their performance tends to capture an even lower percentage of the achievable theoretical limit for a given airspeed. Powered aircraft can therefore be said to be significantly LESS efficient as a result of the application of energy. This is a most unwelcome irony which must be solved. A source of energy should result in greater efficiency than possible without it.
Applying power for the purpose of drag reduction, rather than exclusively for the production of thrust, is to utilize the concept of open thermodynamics. In accordance with well-known methods and studies in the art, controlling the boundary layer of airflow in contact with the aircraft by means of power greatly assists in achieving the GvK limit. Yet many impracticalities within prior art have kept this and other useful concepts from commercial application. Drag reduction—into the range mandated for true efficiency—has not been practically or economically achieved.
Mastering the subleties presented by lower apparent air viscosity is necessary to achieve true fuel efficiency in transportation. Where our aircraft are of sufficient size and speed to increase apparent viscosity to high levels (high Reynolds numbers), we have become reasonably expert. However, with respect to flight at the speeds most useful for distributed and regional transportation solutions, prior art has failed to acknowledge, let alone reach, the much higher potential of a comprehensive fluid dynamic solution such as provided by the present invention.
Thus, the Gabrielli-von Karman plot of specific resistance reveals a near-total lack of fundamentally efficient conveyance within the speed range between 90 and 400 miles per hour. Bounded by autorail and airship at the low end, and by highly efficient, large jet aircraft above 450 MPH, the conspicuous transportation gap between automobile and airliner is as notable for its persistent lack of fuel efficiency as for its standard bearer: the fifty year old general aviation aircraft design.
A significant factor limiting the success of prior art is that highly efficient aircraft have tended toward long wingspans of high aspect ratio in pursuit of this need. Among the glider-like designs having high AEI (LID) scores, speed and load carrying capacity are both limited by material strength; whereas both the efficiency index and practical usage favor powered aircraft that reach high efficiency at high speeds. Among aircraft having identical LID, the faster vehicle will be closer to the GvK limit, thus more efficient.
Fundamentally, a need exists in the art for achieving minimum fluid disturbance in low-viscosity fluid-borne flight.
Finally, practical roadable and stowable aircraft are needed. New technology in aircraft design should give greater priority to removable and foldable flight surfaces to simplify ground transport and storage. Invention that builds from a base of simplicity, safety and efficiency in these requirements leads the way to practical flying vehicles that may be drivable. Similar mechanical challenge is involved in variable geometry wings. For both cases, simplified control paradigms and light weight are paramount to overcoming the failures of prior art. Extensive study and research into these and the foregoing areas, including flight modeling and scale model testing, has through insight resulted in the exemplary solutions embodied in the present invention.